[Clang] Fix crash in CIndex, when visiting a static_assert without message

After implementation of "[Clang] Implement P2741R3 - user-generated static_assert messages"  (47ccfd7a) the c indexer crashes when handling a `static_assert` w/o any message.
This is caused by using `dyn_cast` to get the literal string, which isn't working on `nullptr`.

Reviewed By: cor3ntin

Differential Revision: https://reviews.llvm.org/D156053